ECHINOIDEA 571 



remains in position and extends over the region vacated by the peri- 

 proct. In most of the irregular urchins (though not in certain primi- 

 tive forms known as Holectypoida or Protoclypeastroida) the aboral 

 parts of the ambulacra are expanded to an oval shape (petaloid) and 

 bear flattened, respiratory tube feet. These peculiarities are associated 

 with the habit, possessed by typical members of both orders, of living 

 partly or wholly buried in sand (see below). 



Order ENDOCYCLICA 



Echinoidea in which the mouth is central, the anus remains within 

 the apical area, and the ambulacra are not petaloid. 



Echinus (Figs. 422-428). A typical example, described above. 



Order CLYPEASTROID A 



Echinoidea in which the mouth is central and furnished with a 

 lantern, the anus outside the apical area, the dorsal parts of the am- 

 bulacra nearly always petaloid, and the body usually much flattened. 

 The members of this order live at or near the surface of the sand, 

 and walk by means of the tube feet, which are very numerous. They 

 extract food from the sand, which they shovel into the mouth by 

 means of the teeth. 



Clypeaster. A typical member of the group, of large size, wide- 

 spread in tropical waters. 



Echinocyamus. Small, oval, and not extremely flattened. E.pusillus 

 is a British species. 



Order SPATANGOIDA 



Echinoidea in which the anus and often also the mouth are excentric, 

 the lantern has disappeared, the dorsal parts of the ambulacra are 

 petaloid, and the body cushion-shaped or heart-shaped. 



Typical members of this order live buried at some depth in the 

 sand and move, not by means of their tube feet, but by ploughing their 

 way with numerous, curved, flattened spines. In such forms the body 

 has a heart-shape, owing to the depth of the anterior ambulacrum, 

 which differs from the rest and has special tube feet, capable of great 

 elongation and provided with fringed discs. These gather sand rich 

 in food, which is then pushed into the mouth by stout buccal tube 

 feet. 



Spatangus and Echinocardium (Fig. 429) are typical members of 

 the order, found in British waters. Echinocardium comes into shallower ' 

 water than Spatangus, burrows deeper, and differs in respect of the 

 arrangement of the spines. 



